1. Field of the Invention
The present invention relates to an aluminum nitride-based sintered body having high thermal conductivity which is produced by baking at a relatively low temperature. More particularly, it relates to an aluminum nitride-based sintered body which comprises as a sintering additive a combination of a compound of a Group IIa metal and a compound of a Group IIIa metal of the Periodic Table, and also to a method of producing the same.
2. Description of the Prior Art
As a result of the progress in large-scale integration in recent years, there is an increasing demand for insulated substrates having high thermal conductivity in order to mount semiconductor elements generating large amounts of heat such as high-density integrated circuits, power transistors, laser diodes, and the like.
Sintered bodies based on beryllium oxide or beryllia (BeO) have conventionally been used as ceramic materials having high thermal conductivity, but their toxicity prevents their use in a wide variety of applications.
As a substitute for beryllia, aluminum nitride (AlN) has been used in the production of substrates of high thermal conductivity because it is stable and has high thermal conductivity in addition to good high-temperature strength and electrically insulating properties.
Although aluminum nitride possesses properties which make it suitable for use in the production of semiconductor substrates, it suffers from the problem that it is difficult to sinter. Since it is difficult to sinter an aluminum nitride powder alone in order to obtain a shaped article, it is conventionally sintered with the aid of a sintering additive, which is mixed with an aluminum nitride powder prior to shaping. Sintering additives which are known to be suitable for this purpose include compounds of Group IIa metals (alkaline earth metals), Group IIIa metals (rare earth metals), and Group IIIb metals (aluminum group metals) of the Periodic Table such as Y.sub.2 O.sub.3 (yttrium oxide or yttria), CaO (lime or calcium oxide), and CaC.sub.2 (calcium carbide). See Japanese patent Laid-Open applications Nos. 59-207184, 60-60910, 60-65768, 60-71575, 60-127267, 60-178688, and 50-23411, as well as Huseby et al., U.S. Pat. Nos. 4,578,232, 4,578,233, 4,578,234, 4,578,364, and 4,578,365, and Aldinger et al., U.S. Pat. Nos. 4,591,537 and 4,627,815.
However, none of these prior art references discloses that a particular combination of two or more sintering additives provides superior results.
Huseby et al. in the above mentioned U.S. patents employs a complicated process in which free carbon or a carbonaceous material is added to a mixture of the starting powder of aluminum nitride and a sintering additive, and the mixture is shaped and baked. Addition of free carbon or a carbonaceous material is effective in reducing the oxygen content of the sintered body, thereby increasing the thermal conductivity thereof. However, the presence of a substantial amount of free carbon during baking causes a reaction of carbon with a sintering additive to form a carbide, e.g., YC.sub.x. The carbide, in turn, reacts with oxygen and/or moisture in air after baking, thereby causing deposition of Y.sub.2 O.sub.3 on the surface of the sintered body and remarkably roughening the surface. Therefore, the surface of the sintered body has to be abraded prior to use so as to obtain a smooth surface. This adds an extra step of abrasion and deteriorates the water resistance of the sintered body. In addition, a relatively high baking temperature is required in the presence of free carbon, which is disadvantageous from the viewpoint of economy. Japanese patent Laid-Open application No. 61 -117160 discloses an aluminum nitride sintered body comprising as sintering additives a rare earth metal compound and an alkaline earth metal compound. The sintered body is prepared by adding to an aluminum nitride starting compound a rare earth metal compound and an alkaline earth metal compound in a total amount of from 0.01 to 20% by weight calculated as oxides based on the weight of the aluminum nitride powder, followed by shaping and baking. The amounts of the sintering additives present in the sintered body product are not specifically mentioned in the application. It is estimated that the calcium content of the sintered body is extremely low, because it is known that calcium oxide tends to readily vaporize during baking at a high temperature and its content may be decreased to about from one-tenth to one-hundredth of its initial content upon baking depending on the baking conditions. The aluminum nitride sintered body disclosed in the above laid-open application can be prepared by baking at a relatively low temperature of 1700.degree. C. or below and is described as having a high density and a high thermal conductivity. However, the thermal conductivity actually obtained in the sintered body is 110 W/mK at most.
The theoretical thermal conductivity of aluminum nitride is as high as about 320 W/mK. On the other hand, an aluminum nitride sintered body obtained by baking a mixture of aluminum nitride powder and a sintering additive has a thermal conductivity which is, in general, significantly lower than the value expected from the thermal conductivities of aluminum nitride and the sintering additive due to the presence of oxygen and other impurities in the sintered body and the presence of grain boundaries. For example, sintered bodies of aluminum nitride presently available on the market have a thermal conductivity on the order of 100 W/mK or less. Accordingly, there is room for improvement in the thermal conductivity of an aluminum nitride sintered body, and such improvement is highly desired.
There are two well-known methods for preparing aluminum nitride powder. One is the direct nitriding method in which metallic aluminum powder is directly nitrided with nitrogen or ammonia gas. The other is the alumina reduction method in which alumina powder is mixed with carbon and baked in nitrogen or ammonia gas to effect reduction of alumina and nitriding simultaneously.
In the direct nitriding method, the aluminum nitride powder product can be prepared inexpensively because the process is relatively simple compared to the alumina reduction method. Therefore, it is advantageous from the viewpoint of economy to use an aluminum nitride starting powder prepared by the direct nitriding method. However, it is usually contaminated in an amount of at least a few percent by weight with cationic impurities which enter the product from the grinding vessel or grinding media in the step of grinding the starting metallic aluminum material in order to increase the efficiency of nitriding or in the step of pulverizing the aluminum nitride powder formed by nitriding in order to reduce the particle size to one suitable for use in shaping and sintering. The pulverization of the aluminum nitride powder is usually required since a considerable portion of the powder is agglomerated after nitriding. Also in the pulverization of the aluminum nitride powder, the surface of the powder is oxidized to a certain degree, and therefore the aluminum nitride powder product obtained by the direct nitriding method usually contains oxygen in an amount of at least 2% by weight, and in most cases at least 3% by weight. Such aluminum nitride powder containing oxygen and cationic impurities in such relatively large amounts is not suitable for use as a starting material to produce high-quality aluminum nitride sintered bodies. For this reason, there is only limited use for aluminum nitride powder obtained by the direct nitriding method in the production of sintered bodies having high thermal conductivity with the aid of a sintering additive.
In the alumina reduction method, since agglomeration of particles does not occur significantly during nitriding, the starting alumina can be previously pulverized to the desired particle size prior to reduction and nitriding, and the resulting aluminum nitride powder can be used without further pulverization. Thus, according to this method, aluminum nitride powder having an average particle diameter of 2 .mu.m or less can be obtained and it can be directly used as a starting powder in the production of sintered bodies. Because there is no pulverization step after nitriding, aluminum nitride powder prepared by the alumina reduction method is relatively pure. Its content of cationic impurities can be readily decreased to 0.5% by weight or less, and its oxygen content is usually at most 3% by weight. In view of these advantages, in the production of aluminum nitride sintered bodies in accordance with the prior art, aluminum nitride powder obtained by the alumina reduction method has been used in most cases as a starting powder to be sintered with the aid of a sintering additive.
However, as mentioned above, most of the aluminum nitride sintered bodies obtained by conventional methods from the above-mentioned relatively pure starting powder prepared by the alumina reduction method exhibit relatively low values for thermal conductivity on the order of 100 W/mK or lower.